Fuels used in diesel engines tend to produce exhausts containing undesirable particulates, both visible and invisible. For example, when the diesel engine is operated under heavy loads or when it is a worn or dirty engine, visible emission of smoke often occurs. Small particulates, invisible to the naked eye, can be and usually are emitted even when the combustion chamber and exhaust system of the engine are clean and the engine is operated under normal loads.
It is of course important to minimize the extent of such particulate emissions as much as possible. Some of the prior work along these lines is described in U.S. Pat. Nos. 3,817,720 and 4,378,973.